Ongoing technology and standardization developments make the use of large antenna arrays at cellular base stations and other wireless access points a viable option to boost the air interface capacities and maximum data rates of wireless communication networks. Consider a base station or an access point equipped with a large number of antennas. The node can simultaneously schedule multiple wireless devices in the same time/frequency band, using simple linear processing such as maximum-ratio transmission or zero-forcing in the downlink and maximum-ratio combining or zero-forcing in the uplink. Current literature often refers to these multi-antenna arrangements as very large multiple-input-multiple-output, VL-MIMO, or as “massive” MIMO. VL-MIMO systems are also sometimes referred to as “full dimension” or FD systems. FD-MIMO provides throughput gains without consuming additional spectrum and further offers substantial improvements in radiated energy efficiency. Reflecting the burgeoning interest in FD-MIMO technology, the Third Generation Partnership Project, 3GPP, has an active work item focused on the use of FD-MIMO.
Narrow beam forming in the downlink represents a key aspect of FD-MIMO. Base stations use narrow beam forming to focus transmitted energy towards desired users—i.e., towards the wireless devices being served at any given time. Focusing the radiated energy boosts coverage and raises the maximum data rates achievable on the downlink under real-world channel conditions.
Accurate channel state information, CSI, is a requisite for effective beamforming and acquiring accurate CSI in a scalable fashion for FD-MIMO systems is non-trivial. In conventional systems, radio network nodes transmit per-antenna pilot signals, and wireless devices estimate downlink channel gain based on measurements of the pilot signals. These per-antenna approaches are not feasible for a base station that uses a large number of downlink transmit antennas.
Where reciprocity exists between the uplink and downlink channels, such as in Time Division Duplex, TDD, operation, a wireless device transmits a Sounding Reference Signal, SRS, or other type of reference signal on the uplink. The receiving network base station uses the received reference signal to estimate both the uplink and downlink channels between it and the wireless device. For the channel estimation to be of sufficiently high quality, the base station must receive the uplink reference signal(s) with a sufficiently high Signal-to-Noise Ratio or SNR. This requirement poses challenges for the typical wireless device, which generally is battery operated or otherwise power-limited. Because the uplink should be sounded over the entire frequency band of interest, potentially significant energy radiation by the wireless device is required to achieve sufficient signal quality at the base station. Operation by a wireless device within a cell edge region exacerbates the problem of providing the network base station with reference signals of sufficiently high reception quality.
Beamforming represents one trick available to multi-antenna wireless devices for ensuring that the network base station receives its uplink reference signals at a sufficient received-signal strength. With uplink beamforming, more of the radiated signal energy is steered towards the network base station, thereby improved received signal quality at the network base station for the uplink reference signal.
However, this disclosure recognizes that several disadvantages or problems attend the use of beamforming for uplink reference signal transmission. For example, the network base station uses the received reference signal to estimate the downlink channel to the wireless device, e.g., for link adaptation of its downlink transmissions to the wireless device. Taking the exact same propagation channel conditions, the uplink channel as perceived by the network base station will look different in depending on whether or not the wireless device uses beamforming for its uplink reference signal transmission. Further, beamforming characteristics depend on antenna weights used by the wireless device for beamforming. Consequently, the perceived or “effective” channel seen by the network base station depends on the particular precoder—antenna weight matrix—used by the wireless device.
Providing information to the network base station regarding uplink beamforming solves these problems. However, that approach requires additional uplink signaling from the wireless device and, therefore, represents an added signaling burden.